Elemental phosphorus comes in four forms: white phosphorus, red phosphorus, purple phosphorus, and black phosphorus. White phosphorus is very reactive and will spontaneously combust (burst into flames) when exposed to warm air. Therefore it is usually stored under water. The other forms are relatively nonreactive. One compound of phosphorus, calcium phosphate (Ca3(PO4)2), is a major component of bones. Another, ATP, is used by cells to produce energy.

Phosporus is used in fertilizers and detergents. Phosphoric acid (H3PO4) is used to make soft drinks. Some phosphorus compounds are used to make light bulbs and television sets. Drinking water with phosphorus or taking phosphorus pills became fashionable and was thought to increase brain activity and make you smarter in the mid-1800s when phosphorus was discovered in the brain. In an ironic twist, phosphorus was used by the Allies in World War II to make their bombs glow bright during nighttime raids over Germany, the country of its initial discovery almost 300 years earlier. Phosphorus is also used to make nerve gas and inside glow sticks.

White phosphorus is very toxic and is very damaging to human tissues, especially bones and cartilage. White phosphorus will spontaneously combust (burn) if exposed to air warmer than 35 degrees Celsius. Red, purple, and black phosphorus are relatively safe to handle.